Radar systems are often confronted with the problem of detecting moving targets in the presence of more-or-less stationary clutter. This problem is attacked from two fronts. One solution to this problem is to utilize the narrowest possible antenna beam and the widest bandwidth to minimize the radar cell size, i.e., the physical volume from which clutter echoes are being received at any instant of time. The other solution is to employ methods of comparing the phase of successive echoes to suppress stationary echoes which is commonly referred to as the MTI (moving target indicator) technique. MTI, however, is hampered by restrictions on a number of radar parameters. The use of a uniform pulse repetition frequency (prf) creates blind velocities at which the target moves an integer number of half wavelengths between pulses. To counter this particular problem, some systems have a staggered prf, but it greatly adds to the complexity of the system. The prf is rarely as high as would be desirable from the standpoint of velocity discrimination. It is limited by the range ambiguities it creates. As a result, the number of pulses received during the time that the beam of a search radar scans past the target is limited. Systems which must make a target detection based on very few samples of data are inherently vulnerable to intermittent interference from other radars or from jammers; therefore, hits per beamwidth is an important parameter which restricts the degree to which the beam can be narrowed.
A characteristic desired by most modern radars is frequency agility which is the ability to switch between a number of transmission frequencies as the beam passes over the target. This is desirable to reduce the probability of prolonged interference from other radars, narrow banded jammers, etc. Examples of such radars are shown and described in the following patents: U.S. Pat. No. 3,229,284, issued to W. L. Rubin; U.S. Pat. No. 3,263,227 issued to J. W. Ferry et al; and U.S. Pat. No. 3,267,467, issued to L. Gerardin et al. The Rubin patent describes a radar adapted for the transmission of a linear frequency modulated pulsed carrier signal. The Ferry et al patent describes a radar system where the transmitted pulse is made up of a number of pulse segments each of equal duration but of different frequency, transmitted sequentially and where the received return pulse signals are added to form a composite pulse whose width is that of the duration of one of the transmitted pulse signals. The Gerardin et al patent describes a radar wherein the transmitted pulse is sub-divided into a certain number of constitution pulses each of which have a different frequency varying according to a predetermined order; however, the order is the inverse of that in which they are emitted in the previous composite pulse.
Frequency agility is also extremely beneficial in the clutter environment. Frequency agility causes pulse-to-pulse scintillation of a strength of the echo from both target and clutter rather that the usual scan-to-scan variation. False alarms created by clutter are reduced, because the average of the echoes received during one beamwidth motion of the antenna does not fluctuate so wildly. Detection of targets is also improved when one is concerned with high detection probabilities because abnormally low target cross-sections at certain frequencies are averaged with higher values at other frequencies.
Most of the improvement in target detectability is achieved by the time the number of frequencies per beamwidth reaches ten but false alarm improvement continues indefinitely. It is desirable, therefore, to radiate a minimum of ten frequencies as the radar scans pass the target. In a clutter environment the ability to detect targets is determined solely by the number of frequencies one is able to sample in one beam width and not the amount of energy radiated.